1. Field of the Invention
The present invention relates to an image sensing apparatus and an image capturing system.
2. Description of the Related Art
Recently, image sensing apparatuses having 10,000,000 pixels or more are widely used. In movie shooting using such an image sensing apparatus, the time required for reading out signals from all pixels of the pixel array is long because of the large number of pixels. This decreases the number of sensed images per second (lowers the frame rate).
To prevent this, a technique has been proposed in which the time of signal readout from the pixel array is shortened by skipping and without reading out signals from some of the pixels of the pixel array (performing thinning-out readout) (Japanese Patent Laid-Open No. 9-214836). This technique makes it possible to increase the number of sensed images per second (increase the frame rate).
On the other hand, there is a demand for widening the dynamic range of an image signal in sensing an object. Particularly when an object has a high-luminance portion, electric charges accumulated in pixels corresponding to the high-luminance portion may be saturated. More specifically, a signal read out from the pixel array may reach the upper limit of the dynamic range and become saturated.
A technique has been proposed to solve this problem in which each pixel of the entire pixel array performs a long-time charge accumulation operation and a short-time charge accumulation operation, and a signal obtained by the long-time charge accumulation operation and that obtained by the short-time charge accumulation operation are synthesized (Japanese Patent Laid-Open No. 2001-346096). This technique enables to widen the dynamic range of a signal read out from the pixel array.
Another technique has been proposed, which executes a first electronic shutter operation for each pixel in a specific pixel region at part of the pixel array and then a second electronic shutter operation for each pixel in the region except the specific pixel region of the pixel array (Japanese Patent Laid-Open No. 2003-143485). This technique allows changing the charge accumulation period of the pixels between the specific pixel region and the region other than the specific pixel region.
The image sensing apparatus of Japanese Patent Laid-Open No. 9-214836 displays a partial image at a high resolution without performing thinning-out readout from the pixel region of interest while displaying the whole image at a low resolution by performing thinning-out readout from the entire pixel array.
Japanese Patent Laid-Open No. 9-214836 does not disclose the charge accumulation period of the entire pixel array and that of the pixels in the pixel region of interest to obtain the whole and partial images.
On the other hand, the techniques disclosed in Japanese Patent Laid-Open Nos. 2001-346096 and 2003-143485 read out signals from all pixels of the pixel array. If the technique of Japanese Patent Laid-Open No. 2001-346096 or 2003-143485 is applied to an image sensing apparatus having 10,000,000 pixels or more, the frame period for obtaining the signals of the whole image may be very long, and it may be impossible to achieve the frame rate necessary for movie shooting. That is, it is difficult to increase the frame rate in reading out a signal from the pixel array.
Japanese Patent Laid-Open No. 2001-346096 aims at reducing the size and cost of an image sensing apparatus, and the number of pixels of the image sensing apparatus tends to be smaller. For this reason, it is possible to achieve a sufficient frame rate without performing thinning-out readout in reading out a signal from the pixel array of the image sensing apparatus for movie shooting.
In Japanese Patent Laid-Open No. 2003-143485, signals by electric charges accumulated in the pixels of the pixel array are read out dot-sequentially. Such an arrangement supposedly requires a long time for the signal readout operation, and the number of pixels of the image sensing apparatus needs to be small for movie shooting. For this reason, if the number of pixels is small, it is possible to achieve a sufficient frame rate without performing thinning-out readout in reading out a signal from the pixel array of the image sensing apparatus for movie shooting.